Does My Late 80s F-150 Need a New Fuel Tank Sending Unit?
The fuel tank sending unit on a vehicle, including a Ford F-150, is a simple but relatively delicate item. The sending unit works by using varying levels of electrical resistance to a small charge being sent through a circuit. The differing resistance levels tell the vehicle’s computer what the fuel level in the tank is. The resistance is determined by the position of a small float that raises and lowers as the fuel in the tank increases or decreases.
It’s not uncommon for high-mileage vehicles, especially older vehicles like a late-80s F-150, to begin to have problems as corrosion and wear begin to affect the sending unit. Unfortunately, replacement of the sending unit is not easy. It requires removing the fuel tank and then removing the fuel pump and sending unit for replacement. For most home mechanics, especially on a truck that’s been around for 30 years, the job will require a lot of extra effort to get around rust and grime buildup.
Diagnosing the Sending Unit
The symptoms of a failing or failed fuel tank sending unit are fairly straightforward. If the fuel gauge reads only intermittently or not at all, you likely have a sending unit issue. Most 1980s F-150s are equipped with two fuel tanks, usually referred to as “front” and “rear” with a toggle switch that allows the driver to choose between those tanks. The rear tank, located above the spare tire and spanning the width of the bed’s framing, is a bit easier to get to. The forward tank, located beneath the forward fuel filler port along the driver’s side, requires a little more work to access.
Because it is possible to switch between tanks, a faulty sending unit is easier to test. If the fuel gauge works okay with one tank but not the other, it’s obviously a sending unit issue. If it doesn’t work with either tank, then testing on the fuel circuit should be done. There is the potential that both sending units are bad, but this is far less likely than the circuit itself being broken or shorted.
Replacing the Sending Unit - Starting Out
Once it’s been determined that it’s the sending unit at fault, replacement is fairly simple, though it requires a fair amount of time and work. Most of this will be because the fuel tank must be removed in order to access and replace that unit. A replacement sending unit for the front tank can be found here, rear tank here.
The removal process will differ slightly between the front and rear tanks, but in general, it requires the following:
- Removing protective skid plates
- Unstrapping the tank to lower it enough to gain access to the lines and wiring
- Disconnecting the fuel lines, return line, and fuel pump/sending unit wiring harness
- Finally, lowering the tank completely for removal
When working with the fuel system, safety is key. Be sure to take these precautions:
- Drain the tank as much as possible before removing it
- Have proper safety equipment on hand such as a fire extinguisher capable of putting out fuel (chemical) fires
- Eye protection is a great idea whenever working with a part of your vehicle that has rust
- Work in an open, well-ventilated area
Fuel fumes are not only toxic but highly flammable. Fires are easy to start and difficult to put out. Having a helper is highly recommended, even if just for safety reasons.
Replacing the Sending Unit - Finishing Up
Once the tank is lowered and removed, the sending unit is attached to the fuel pump, which is inside the tank. The wiring harness and fuel line point will be surrounded by a spanner nut, which can be removed by turning it counterclockwise with a spanner wrench or by carefully using a screwdriver to push the ring. As with most of the other bolts on your older F-150, there is likely a lot of rust involved, so using a wire brush and penetrating oil will greatly simplify the job and make things both safer and cleaner.
Once the old pump and sending unit are lifted out, the new unit can be put in place. Be sure to test it thoroughly before installation to be sure that it’s both the right part and that it’s operational. You can test using either an ohm meter or by plugging it into the truck’s wiring harness and turning the key to the “on” position (without starting the truck). Immediately on the key’s turn, the pump should activate for about a second, manipulating the float position which should result in changes to the fuel level on the dashboard’s gauge. Be sure to turn the key off and disconnect the negative battery cable before re-installing the fuel tank with the new unit inside.
On a well-maintained and relatively clean 1980s Ford F-150, a home mechanic should be able to replace the fuel pump and sending unit in about three hours.
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